1. Field of the Invention
The present invention relates generally to an apparatus and a method for interference cancellation in a wireless communication system, and more particularly, to an apparatus and a method for controlling a power to satisfy Quality of Service (QoS) per user in interference cancellation in a multi-antenna system.
2. Description of the Related Art
Rapid growth of a wireless mobile communication market requires various multimedia services in a wireless environment. To provide the multimedia services, research is being conducted on a multi-antenna system (e.g., Multiple-Input Multiple-Output (MIMO) system) for efficiently utilizing the limited frequency in accordance with the mass transmit data and the high data rate.
The multi-antenna system transmits data using an independent channel per antenna, and thus enhances the transmission reliability and the data rate without an additional frequency or transmit power allocation, compared to a single-antenna system. Also, the multi-antenna system can raise the spectral efficiency such that multiple users simultaneously share the spatial resources acquired through the multiple antennas.
The multi-antenna system in the multi-user environment suffers interference between the users or between the antennas. To mitigate the interference, the multi-antenna system adopts a linear scheme such as a Zero Forcing (ZF) beamforming scheme or a Minimum Mean Square Error (MMSE) scheme.
When the multi-antenna system cancels interference using the linear scheme, the implementation of the multi-antenna system is easy to facilitate, and the error rate does not greatly increase even when available channel information is less than the entire channel information. However, in using the linear scheme, the multi-antenna system cannot acquire the maximum channel capacity by means of the multiple antennas.
Hence, the MIMO system of the multi-user environment cancels the interference between the users or the antennas using a Dirty Paper Coding (DPC), which is a nonlinear precoding scheme. Using the DPC in the multi-antenna system of the multi-user environment, the transmitting end sends a transmit signal by subtracting an integer of the amount considering the interference from the transmit signal through the modulo operation. The receiving end can infer the information through the same modulo operation as in the transmitting end. For example, the multi-antenna system can cancel the interference using either Tomlinson-Harashima Precoding (THP) which applies the DPC in one dimension, or Vector Perturbation (VP) which applies the DPC in N dimensional vectors through the modulo operation.
As discussed above, the multi-antenna system of the multi-user environment can alleviate the interference between the users or the antennas using the linear or nonlinear scheme.
However, the linear scheme is likely to degrade the performance of the multi-antenna system and to waste the transmit power unless the channels selected for the signal transmission between the users are orthogonal. The nonlinear scheme demands the constant transmission rate per user in the multi-antenna system and does not guarantee Quality of Service (QoS) per user because of the DPC.